1. Field of the Invention
The invention relates to a device for the accelerated photo-aging of materials containing polymers, which device enables measurement of their capability to resist the simultaneous action of ultraviolet radiation, temperature and oxygen in an aqueous or gaseous phase in order to analyze with a view to better understanding the phenomena of photodegradation and to determining the correlations between the life spans of said materials under accelerated photo-aging and under climatic aging.
2. Background of the Related Art
It has been known for a long time that polymers exposed to natural climatic conditions undergo over time an alteration in their chemical nature which is manifested, for example, by a modification in their surface appearance (discoloration, loss of brilliance, becoming powdery, etc.) most often accompanied over time by a degradation of their mechanical characteristics (resistance to traction, stretching, shock resistance, rigidity).
To better comprehend the overall phenomena occurring during exposure of said polymers to natural climatic conditions, numerous simulation devices have been proposed to enable the reproduction in an accelerated manner of the effects noted naturally on said materials.
An accelerated photo-aging device for polymers is known from French Patent No. 2 430 609, which comprises chamber in which a single lamp emits a narrow parallel beam of ultraviolet radiation which irradiates a group of test pieces mounted on a sample holder, which group is reciprocally moved in translation in a plane perpendicular to the axis of the beam. The temperature of the chamber is regulated so as to maintain constant the temperature of the test pieces, by means of temperature sensors in direct contact with said test pieces.
Such a device has disadvantages which are bothersome in use. Firstly, the polymer test pieces are submitted only to localized and discontinuous exposure to ultraviolet radiation due to the reciprocating movement of the sample holder. In addition, due to its construction, this device allows the simultaneous irradiation of only a small number of test pieces. Finally, such a device enables irradiation only in a dry atmosphere and, consequently, does not enable the study of the phenomena of photodegradation in the presence of an aqueous phase.
However, the accelerated photo-aging of polymers must necessarily be representative of the most extreme climatic phenomena, that is, not only the physical aggressions due to natural light and variations in temperature must be taken into account, but also the chemical aggressions due to the presence of an aqueous medium and/or oxygen such as occurs when the materials produced from polymers are used in direct contact with these natural elements.
Devices enabling a more complete climatic simulation by means of exposure in a damp medium have been proposed in the specialized literature dealing with the accelerated photo-aging of polymers.
A first type of device comprises an enclosure in which is placed a radiation chamber provided at its central part with three 4.5 kW Xenon lamps arranged in a triangle, with each lamp being provided with selective reflecting flat metal mirrors for ultraviolet light, around which are arranged two coaxial quartz cylinders, comprising an annular volume in which a cooling fluid circulates. Placed between the enclosure and the ultraviolet radiation chamber there is a cylindrically shaped sample holder moved in continuous or alternating rotation. In addition, the device is provided in the upper part of the enclosure with one to three sprinkler ramps fitted with three nozzles enabling the creation of a damp internal atmosphere and/or a sprinkling of water onto the test pieces. Finally, the device contains means enabling the temperature within the enclosure to be regulated by means of the continuous measurement of a black body located therein and to automatically adjust the test piece sprinkling cycles. Such a device is known by the trade name of XENOTEST 1 200 HERAEUS.
Another type of device sold under the trademark HERAEUS XENOTEST 250 T, provided with means for the thermal regulation of the enclosure and means for spraying water and adjusting the cycles of the spraying according to the preceding technique, is provided with a parabola-shaped reflector with a vertical axis, whose source is provided with a low pressure Xenon burner fitted with a filter, with the test pieces being arranged flat on the horizontal floor of the enclosure which thus acts as the sample holder.
In use, both of these devices have major disadvantages.
In order for the phenomenon of degradation of polymers subjected to ultraviolet irradiation to be significant and reproducible, it is first of all necessary for the actual temperature of the samples to be maintained constant throughout the tests and at a known value. However, in the two above-identified devices, the temperature of the polymer samples is not measured in a precise manner because, in each device, the temperature of the enclosure containing the samples is regulated via the measurement of the temperature of a black body placed in said enclosure, which temperature, according to the radiation properties of said body, is always higher than that of the samples. In addition, in the second device, resistance probes are arranged on a small rule placed in direct contact with the sample holder so as to measure, as far as possible, the temperature of the samples. However, because of the differences in temperature due to the imperfection of the thermal contacts between the resistance probes, the sample holder floor and the polymer samples arranged on said floor, the temperature measured by the resistance probes is different from that of the exposed materials.
In addition, the samples of polymer used in the second device are maintained in a fixed position. Since the radiation emitted cannot have a perfectly homogeneous distribution over the entire volume of the irradiation chamber, the result is that the incident light intensity received by the samples during the total time of their irradiation cannot be the same from one sample to another and that heterogeneous distribution of said intensity can cause inaccurate aging test results.
Finally, a more specific device has been proposed which consists of a parallelepipedal enclosure on which is placed a prism-shaped assembly provided with eight fluorescent tubes producing ultraviolet radiation placed horizontally, four by four, along two concurrent surfaces of the prism-shaped assembly. The samples to be irradiated are then arranged between the tubes and the two above-identified concurrent surfaces. The space defined by said concurrent surfaces and the samples enables a natural circulation of ambient air for cooling.
In its lower part, this device is also provided with a water vat for the generation of water vapor. The difference in temperature existing between the water vapor and the internal irradiated surface of the samples cooled at their external surface due to the circulation of ambient air is sufficient to cause condensation of that water vapor coming into contact with said internal surface. The thermal gradient which is established within the thickness of the sample does not enable the temperature of the irradiated material to be determined. In addition, like the second device described above, the samples placed along the concurrent surfaces of the enclosure are maintained in a fixed position throughout the entire time of the experiments and cannot therefore receive an equal amount of radiation from, one sample to another.
Finally, this device is provided with light sources emitting energy in the range of 270 to 350 nm which includes a photon-rich zone (between 270 and 300 nm) which does not exist in the solar spectrum. The short wavelengths can create unrepresentative phenomena of natural aging in the accelerated laboratory photo-aging tests, such as accelerated photopassivation or photodegradation phenomena.
While the three devices described offer the possibility of enabling the study of photo-aging of samples of polymers subjected to the action of running water or water vapor which condenses directly on contact therewith and while they also enable the simultaneous reproduction of certain water washing effects (physical taking up and/or dissolving of additives or chemical photodegradation products having migrated to the surface of the test pieces) and certain mechanical effects, they do not enable the reproduction and understanding of the chemical role of water during the irradiation nor even its possible role as an extraction solvent. Likewise, it has been noted that, under the photo-oxidizing action of functional organic groups which are particularly sensitive to the action of water and by a hydrolysis reaction, certain materials generate a breakage of the molecular chains accompanied by a degradation of the mechanical properties of said material.
To be significant, a simulation of the effect of the water must be characterized by the presence of a film of water maintained in direct and permanent contact with the polymer throughout the irradiation, with the oxygen concentration of said water being controlled and maintained at a constant value. However, the devices described above do not enable the presence of a film of water of a given thickness over the entire surface of the irradiated test piece to be permanently ensured.
Thus, a test piece produced using a thin strip of very hygroscopic polyamide, subjected in a sequential manner to sprinkling by spraying in a photo-aging enclosure brought to a temperature of between 40 and 70.degree. C. has a dry surface a few minutes after the end of the sprinkling.